a heart arrhythmia is an abnormal electrical impulse in that heart that contracts the heart in a abnormal way of its function , examples of arrhythmias such as SVT super ventricular-tachycardia which is a rapid heart rate over 100 and is originated under the SA node which creates a reentrant in the AV node atrial ventricular node. so far SVT is not fatal but a consisted SVT for long periods of time can be harmful and produce complications. fatal Arrthymias known as V-tach ventricular tachycardia which basically means that the lower ventricle chambers are rapidly contracting and not letting enough blood flow through the heart lungs and brain this is the cause of being in shock and losing 100% blood pressure which than causes the heart to go in V-fib or induced cardiomyopathy . but there for these arrhythmias are rare on young and healthy adults , V-tach are also commonly formed in the ventricular depolarization when a PVC occurs in the middle of a T-wave in the EKG which is V depolarization.
It is an abnormal cardiac rate or rhythm (arrhythmia). The condition is caused by a failure of the sinus node to maintain its pacemaker function or by a defect in the electrical conduction system. Examples of arrhythmia include:
Bradycardia (slow heart rate),
Ectopic beat (Premature Atrial or Ventricular Contraction)
Tachycardia (a very fast heart rate) examples of this, to name a few...Supraventricular which is a tachycardia arising in the top two chambers of the heart, Ventricular Tach which is a tachycardia originating from the bottom two chambers in the heart.
Hope this helps to shed some light on your question.